There are an estimated 100 million mines and countless millions of acres of land contaminated with unexploded ordnance (UXO) worldwide. Thus, there is a need for sensor systems and methods that can detect and identify large and small metal objects buried in soil. Such objects often are mines and UXOs.
A commonly used sensor for mine and UXO detection is the electromagnetic induction (EMI) metal detector. Conventional EMI metal detectors use either frequency-domain (FD) or time-domain (TD or pulse) eddy current methods and can detect small metal targets (such as plastic-cased low-metal content mines) at shallow depths and large metal targets (such as metal-cased high-metal content mines and UXOs) at both shallow and deep depths under a wide range of environmental and soil conditions. However, metal non-mine (i.e., clutter) objects commonly found in the environment pose a major problem in identifying mines. That is because these clutter objects create false alarms when detected by a metal detector. For time-efficient and cost-effective land clearing, the detected metal targets must be classified as to their threat potential: mine, UXO or clutter. Preferably, these metal targets need to be classified in real-time or near real-time.
FIG. 1 is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or saw-tooth pulsed waveform, or a combination of different positive and negative current ramps.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target 12. This EMF induces eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
Pulse induction metal detector (PIMD) antennas (transmitter and receiver coil) come in two basic types as shown in FIGS. 2a and 2b. The first type of PIMD shown in FIG. 2a illustrates a single combined transmitter and receiver coil 23 and damping resistor 22 with multiple loops of wire forming the coil 23. A current pulse is sent through the multiple turn coil 23 and the received metal detection signal is sensed by the same coil 23. The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier 25 (typical gain factor of 100 to 1000). A protection circuit 24 is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil).
The second type of PIMD illustrated in FIG. 2b uses a separate coil 27 and damping resistor 26 for the transmitter and a coil 29 and damping resistor 28 for the receiver. This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil 29 (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier 25 also sees the high kick-back voltage pulse generated by switching the transmitter coil 27 off abruptly and protection circuitry 24 is needed to protect it from damage.
Typically, for low-metal mine detection the transmitter and receiver coils (commonly called the antenna) are concentric and are about 8″ to 12″ in diameter. This antenna size is to facilitate the portability of the metal detection sensor (e.g., light weight) and to aid in localizing the detected metal (small detection footprint). A smaller antenna size limits the detection depth and a larger antenna size hinders the portability and localization capability of the metal detection sensor for small metal objects.
Metal target classification places additional constraints on a metal detector antenna design. For small metal classification, the above antenna size is appropriate since the low-metal mine metal targets are much smaller than the antenna size (typically, less than 0.25″). For example, the metal object is fully illuminated (with a relatively uniform magnetic field) by the transmitting magnetic field coil and the re-radiated magnetic field is fully received by the receiver coil. The exact placement of the antenna over the metal object is not critical since the transmitter and receiver magnetic field uniformity is relatively large. However, when metal objects are encountered that are on the order of or larger than the metal detector antenna, the metal signature from the detected magnetic field is not unique and can confound a metal classification process. The random placement of the metal detector antenna over a large metal object excites the metal object with different magnetic field vector angles and strengths. The resulting magnetic field signatures are then dependent on the exact placement and orientation of the antenna and the unknown metal object. Since the relative positional information is not known for a hidden or buried object, classification of the metal object is difficult.
FIG. 3 shows the basic configuration of a differential receiver coil for a pulse inductive metal detector (PIMD). These antennas are generally larger than the above single transmitter/receiver coil antennas to allow for a relatively large receiver coil to be placed inside the single transmitter coil. The configuration of the receiver coil is known by different names like a FIG. 8, a gradiometer, or a balanced coil. There are many advantages to a differential receiver coil arrangement. In low-metal content mine detection the major advantage of the differential receiver coil configuration is passive ground return cancellation which allows the metal detector to sense small metal objects in the presence of mineralized soil.
Another advantage of the differential receiver coil is the decoupling of the transmitter pulse from the receiver signal which allows for a faster responding metal detection signal (high bandwidth). A disadvantage of the differential coil arrangement is the need for placing two receiver coils within a given transmitter coil dimension. This naturally limits the receiver coil size and hence the detection depth (metal detection sensitivity and detection depth are functions of the transmitter and receiver coil size). This size constraint does not adversely effect a typical metal detector for landmines since small metal objects characterized by low-metal mines are generally placed within the top few inches of the ground. However, when a large metal object such as a UXO or high metal content mine is encountered with a differential configured receiver coil, the size of the object is sometimes as large or larger than the receiver coil. The receiver coil does not measure the time decay signature of the object correctly for classification purposes. Large objects do not necessarily need the ground balancing of the differential coil arrangement (e.g., large metal object signature is much larger than ground response signature) nor the other advantages convened by the differential coil arrangement (large metal objects have a lower bandwidth compared to small metal objects). Large metal objects are sometimes buried deep in the ground and a differential receiver coil arrangement (for a given transmitter coil size) has relatively poor depth sensitivity.
Thus, there is a need to combine the advantages of a differential receiver coil antenna design (for small metal objects) with a single receiver coil design (for large metal objects) into one metal detection device. By combining the features of both receiver coil configurations into one device we overcome the limitations of each device separately.